1. Field of the Invention
The present invention generally relates to an optical multilayer film and an optical semiconductor device including the optical multilayer film.
2. Description of the Related Art
Conventionally, optical semiconductor devices such as semiconductor lasers and optical modulators have been developed in the form of metal or ceramic packages that are sealed to secure the airtightness of the inside of each device. However, those packages are costly and the sealing procedures are complicated, resulting in difficulty of providing inexpensive products.
To solve this problem, a method of sealing an optical semiconductor device with resin or the like has been recently suggested. By this method, however, the refractive index of the material that covers the optical semiconductor device as an optical element is changed after the sealing is performed with resin or the like. As a result, the optical semiconductor device cannot exhibit desired optical characteristics.
Japanese Unexamined Patent Publication No. 2001-42169 discloses a technique that solves the above problems. In this prior art, a semiconductor device having a single-layer reflection preventing film is sealed with resin, and light reflected by the boundary plane between the reflection preventing film and the resin is damped (or cancelled) by light that propagates in the reflection preventing film. In this manner, deterioration in the optical characteristics of the optical semiconductor device can be restrained.
Meanwhile, it has been widely known that an optical film such as a reflection preventing film is formed by a multilayer film to obtain a higher transmission rate. Japanese Unexamined Patent Publication No. 7-113901 discloses such a structure. In this prior art, however, there are no disclosures concerning a technique to improve and stabilize the optical characteristics of an optical semiconductor device.